SQL As Understood By SQLite

ANALYZE

The ANALYZE command gathers statistics about tables and
indices and stores the collected information
in internal tables of the database where the query optimizer can
access the information and use it to help make better query planning choices.
If no arguments are given, all attached databases are
analyzed. If a database name is given as the argument, then all tables
and indices in that one database are analyzed.
If the argument is a table name, then only that table and the
indices associated with that table are analyzed. If the argument
is an index name, then only that one index is analyzed.

The default implementation stores all statistics in a single
table named "sqlite_stat1". If SQLite is compiled with the
SQLITE_ENABLE_STAT3 option and without the SQLITE_ENABLE_STAT4
option, then additional histogram data is
collected and stored in sqlite_stat3.
If SQLite is compiled with the
SQLITE_ENABLE_STAT4 option, then additional histogram data is
collected and stored in sqlite_stat4.
Older versions of SQLite would make use of the sqlite_stat2 table
when compiled with SQLITE_ENABLE_STAT2 but all recent versions of
SQLite ignore the sqlite_stat2 table.
Future enhancements may create
additional internal tables with the same name pattern except with
final digit larger than "4".
All of these tables are collectively referred to as "statistics tables".

The content of the statistics tables can be queried using SELECT
and can be changed using the DELETE, INSERT, and UPDATE commands.
The DROP TABLE command works on statistics tables
as of SQLite version 3.7.9.
The ALTER TABLE command does not work on statistics tables.
Appropriate care should be used when changing the content of the statistics
tables as invalid content can cause SQLite to select inefficient
query plans. Generally speaking, one should not modify the content of
the statistics tables by any mechanism other than invoking the
ANALYZE command.
See "Manual Control Of Query Plans Using SQLITE_STAT Tables" for
further information.

Statistics gathered by ANALYZE are not automatically updated as
the content of the database changes. If the content of the database
changes significantly, or if the database schema changes, then one should
consider rerunning the ANALYZE command in order to update the statistics.

The query planner loads the content of the statistics tables
into memory when the schema is read. Hence, when an application
changes the statistics tables directly, SQLite will not immediately
notice the changes. An application
can force the query planner to reread the statistics tables by running
ANALYZE sqlite_master.